14 CFR § 91.531 - Second in command requirements.
(a) Except as provided in paragraph (b) of this section, no person may operate the following airplanes without a pilot designated as second in command:
(1) Any airplane that is type certificated for more than one required pilot.
(2) Any large airplane.
(3) Any commuter category airplane.
(b) A person may operate the following airplanes without a pilot designated as second in command:
(1) Any airplane certificated for operation with one pilot.
(2) A large airplane or turbojet-powered multiengine airplane that holds a special airworthiness certificate, if:
(i) The airplane was originally designed with only one pilot station; or
(ii) The airplane was originally designed with more than one pilot station, but single pilot operations were permitted by the airplane flight manual or were otherwise permitted by a branch of the United States Armed Forces or the armed forces of a foreign contracting State to the Convention on International Civil Aviation.
(c) No person may designate a pilot to serve as second in command, nor may any pilot serve as second in command, of an airplane required under this section to have two pilots unless that pilot meets the qualifications for second in command prescribed in § 61.55 of this chapter.